(1) Field of the Invention
The present invention relates to a method of and an apparatus for controlling the supercharge pressure of a turbocharger in which an excessive overshoot condition by the actual supercharge pressure is prevented.
(2) Description of the Prior Art
A turbocharger is constructed in such a manner that an exhaust gas turbine is rotated by exhaust gas, which is at a high temperature and at a high pressure energy so that the pressure within the intake manifold can be increased above atmospheric pressure as the number of rotations or rotational speed of a compressor in the turbocharger increases. As a result, the supply of a large quantity of intake air flow to the engine becomes possible by the supercharge pressure thus obtained, with the result that high torque, high output power and improvement of fuel consumption can be obtained.
Now, in a car engine having a wide range of engine speeds, it is possible to sufficiently secure the supercharge pressure in the middle and high speed operating zones in view of the characteristics of the turbocharger. In the low speed operating zone, however, as it is difficult to obtain a sufficient exhaust pressure, the number of rotations or rotational speed of the exhaust gas turbine can not be increased. As a result, the supercharge pressure is lowered and the torque at low speed will tend to be insufficient. In this case, it is known that the drive efficiency of the exhaust gas turbine is determined in accordance with the ratio of A/R, where A indicates the cross-sectional area of a scroll of the turbine, which is an inlet portion of the exhaust gas entering into the turbine, and R indicates the radius from the center of the scroll.
To this end, a turbocharger of the variable capacity type which has capacity changing means, with the ratio A/R of the turbine being variable, has already been proposed by the same applicant of this application (see, for instance, Japanese Patent Application S.N. 58-162918) in which a supercharge pressure can be obtained even when the turbocharger of the variable capacity type is operated in the low speed operating zone.
The capacity changing means for either increasing or decreasing the opening of the exhaust gas inlet portion to the exhaust gas turbine is driven by an actuator, the working pressure of which is supercharge pressure produced downstream of a compressor. In order to regulate the working pressure, an electromagnetic valve is provided and part of the working pressure is released by the electromagnetic valve to the atmosphere, so as to regulate the pressure. To this end, an electronic control unit including a microprocessor is used and the operation of the electromagnetic valve is controlled by the control unit in accordance with the operating conditions of the engine.
The electromagnetic valve to be utilized in this case, is normally an ON/OFF type electromagnetic valve which operates at a predetermined frequency and its opening time ratio is controlled in accordance with the duty value. For instance, when the duty value representative of the opening time of the valve is 100 percent, it indicates that it is fully opened and the cross-sectional area A is made minimum in this case by means of the actuator and the capacity changing means, so as to increase the rotational speed of the exhaust gas turbine.
On the other hand, when the duty value is zero, it indicates that the electromagnetic valve is fully closed, with the result that the cross-sectional area A is made maximum and the rotational speed of the turbine is suppressed. In actual control of the engine, in order to increase the accuracy of the control, the supercharge pressure is feedback-controlled in accordance with the deviation between the target supercharge pressure and the actual supercharge pressure so as to achieve agreement of the actual supercharge pressure with the target value.
Although a sufficient supercharge pressure can be obtained by the capacity changing means in the low speed operating zone of the engine having a low exhaust gas flow rate, the flow speed of the exhaust gas turbine can not be decreased in a high speed, high load operating zone where the exhaust gas flow rate increases, even when the cross-sectional area A of the scroll is made maximum. As a result, the rotational speed of the exhaust gas turbine increases rapidly and it follows that the rotational speed exceeds its allowable upper limit for the supercharge pressure.
To this end, when the supercharge pressure approaches the upper limit, part of the exhaust gas is bypassed from the upstream of the exhaust gas turbine to the downstream by the provision of an exhaust bypass valve which suppresses the supercharge pressure. Accordingly, when the exhaust bypass valve is opened, the exhaust gas flow into the exhaust gas turbine is decreased, so that the rotational speed of the turbine is also decreased, thus preventing the supercharge pressure from exceeding the upper limit.
In the supercharge pressure control apparatus already proposed by the same applicant of this application, the exhaust bypass valve is also feedback-controlled by detecting actual supercharge pressure as in the case of the capacity changing means, so as to improve the control accuracy of the supercharge pressure. In this case, however, when the capacity changing means and the exhaust bypass valve are both feedback-controlled, it often results in a zone where mutual interference occurs, and the valve of the capacity changing means and the exhaust bypass valve often deviate from their essential optimum valve positions.
For instance, when the supercharge pressure approaches the upper limit in the high speed operating zone of the engine, the capacity changing means is normally fully-opened and there is no reduction in the flow of the exhaust gas, while if the opening of the exhaust bypass valve is adjusted in accordance with the supercharge pressure, the exhaust gas flow is also not increased. As a result, optimum supercharge pressure control can be performed, with the efficiency of the engine operation being maintained in a preferable condition, theoretically. In practice, however, even when the exhaust gas flow speed is increased by reducing the opening of the capacity changing means and the exhaust bypass valve is additionally opened, the same supercharge pressure condition can not be maintained. The reason is that an exhaust gas flow path tends to be unnecessarily reduced by the capacity changing means and this in turn causes the exhaust gas pressure to be increased accordingly, thus lowering the exhaust gas efficiency and lowering the output efficiency as well.
Accordingly, when a feedback control is performed by the capacity changing means and the exhaust bypass valve, a zone to be feedback-controlled is set up in accordance with the operating conditions of the engine. In this case, when any one of the capacity changing means and the exhaust bypass valve is carrying out the feedback control, while the other is maintained constant, any problems occuring from the control interference between the two can be avoided.
Now, in order to increase the acceleration performance of the engine, an over-boost control has already been proposed in which target supercharge pressure is temporarily increased in the accelerated condition so as to keep track of the target supercharge pressure. For the purpose of performing the over-boost control, there are two operations to be carried out in practice; one is to increase the target supercharge pressure when beginning to open the exhaust bypass valve and the other is to increase the flow speed of the exhaust gas by reducing the opening of the capacity changing means at the same time. In the former case, since the rotational speed of the exhaust gas turbine is increased after an increase in the exhaust gas flow rate during engine acceleration, the response for increasing the supercharge pressure is low. On the other hand, in the latter case, the actual supercharge pressure control can promptly be performed as the exhaust gas speed into the exhaust gas turbine is increased.
In this case, however, when the target value of supercharge pressure is increased during acceleration and at the same time when the over-boost control is performed by the capacity changing means so as to rapidly increase the rotational speed of the exhaust gas turbine, the actual supercharge pressure is suddenly increased. However, when the actual supercharge pressure now exceeds the target value, even if an attempt has been made to lower the rotational speed of the turbine, the overshoot phenomena would occur during which the actual supercharge pressure temporarily exceeds the target value largely due to the delay in the operation response of an actuator for driving the capacity changing means.
Accordingly, if the actual supercharge pressure exceeds the upper limit of the target value even if temporarily, excessive air mixed with fuel is supplied into the engine because the target supercharge pressure has been set up at a higher value in the acceleration condition than the normal condition, thus causing a detrimental effect on the durability of the engine due to the excessive load condition.
In order to increase the response described above, when the control gain for the feedback control is increased, a hunting in the control tends to be produced and the control stability lowers.